In this proposal, the effects of testosterone administration alone, or, in combination with exercise, on muscle function in elderly, sedentary men with testosterone deficiency will be investigated. Aging is associated with skeletal muscle sarcopenia and diminished muscle performance, which lead to significant disability from falls and loss of independence. One factor that may lead to sarcopenia is the presence of testosterone deficiency. Serum testosterone levels decrease progressively with age and the majority of elderly men have serum testosterone and free-testosterone levels below the mean for young men. Because testosterone deficiency may lead to alterations in body composition that include decreases in lean mass and strength, it is hypothesized that administration of testosterone therapy to older men with testosterone deficiency (relative to young men) may lead to improvements in muscle function and overall performance. In this proposal, 96 physically inactive elderly men with relative testosterone deficiency will be randomized in a double blind, placebo controlled fashion to receive testosterone enanthate therapy as intramuscular injections at a dose of 200 mg given every 2 weeks versus placebo for a total of 12 weeks. This study will test the hypothesis that administration of gonadal steroids will have a beneficial effect on lean muscle mass, muscle strength, and overall function. These men will be randomized further either to maintain the same level of physical inactivity or to undergo a graded resistance exercise program. This part of the protocol will test the hypothesis that the combined modalities of testosterone and exercise will have additive beneficial effects on muscle mass and function. The goal of these studies will be to determine the efficacy and safety of testosterone administration and, additionally, a home-based exercise program on muscle function in older men with relative testosterone insufficiency.